The fluid-chamber component elements mentioned above may be used, for example, as component elements of a working oil chamber 136, 174, 512 in a stepless speed change gear disclosed by Japanese Utility Model Laid-Open Publication No. 64-41757. Conventionally, such fluid-chamber component elements have been generally made by die casting.
However, such die-cast fluid-chamber component elements present the problem that the component elements have to be made relatively thick in order to have the necessary strength. Further, die casting requires complicated facilities, thus disadvantageously increasing the cost.
As a method which overcomes the problems mentioned above, there has been proposed a method of producing a disk-like piston obtainable by forming a seal ring groove in the peripheral portion of a blank disk using a roller, as disclosed by Japanese Patent Laid-Open Publication No. 63-72441. As shown in FIG. 6, the producing method above-mentioned comprises the steps of holding a blank 100 by and between a pair of rotary molds A', B', thickening the peripheral portion 101 of the blank 100 with a roller C' and forming a seal ring groove 102 in the outer peripheral surface of the peripheral portion 101 thus thickened.
If the blank 100 is thin when producing a fluid-chamber component element by the method mentioned above, the blank 100 is disadvantageously buckled by the peripheral portions of the rotary molds A', B' holding the blank 100 when a pressure is applied to the outer peripheral surface of the blank 100 by the roller C'.